mfd: qcom-spmi-pmic: add support for slow SPMI busses

Add device tree support for configuring if mutexes or spinlocks
should be used in the regmap configuration (i.e. the fast_io
element value).  This ensures that qcom-spmi-pmic slave devices
can be used with SPMI busses that must operate in process
context.

Change-Id: I3abdee36935457db497ce6ff2a242755fc3aff90
Signed-off-by: David Collins <collinsd@codeaurora.org>